1. Field of the Invention
The invention relates to an optical article comprising a substrate coated with a multi-layer transparent anti-reflection (AR) coating having an improved thermal resistance and good abrasion resistance, in particular an ophthalmic lens, and a method of manufacturing such optical article.
2. Description of Related Art
It is a common practice in the art to coat at least one main surface of a lens substrate, such as an ophthalmic lens or lens blank, with several coatings for imparting to the finished lens additional or improved optical or mechanical properties. These coatings are designated in general as functional coatings.
Thus, it is usual practice to coat at least one main surface of a lens substrate, typically made of an organic glass material, with successively, starting from the surface of the lens substrate, an impact-resistant coating (impact-resistant primer), an abrasion- and/or scratch-resistant coating (hard coat), an anti-reflection coating and, optionally, an anti-fouling top coat. Other coatings such as a polarized coating, a photochromic or a dyeing coating may also be applied onto one or both surfaces of the lens substrate.
An anti-reflection coating is defined as a coating, which improves the anti-reflective properties of an optical article when deposited at its surface. It reduces reflection of light at the interface article-air on a relatively wide band of the visible spectrum.
Anti-reflection coatings are well known and classically comprise a mono-layer or multi-layer stack of dielectric materials such as SiO2, SiO, Al2O3, MgF2, LiF, Si3N4, TiO2, ZrO2, Nb2O5, Y2O3, HfO2, Sc2O3, Ta2O5, Pr2O3, and mixtures thereof. They are generally inorganic by nature.
It is also well known that anti-reflection coatings preferably are multi-layer coatings comprising alternatively high refractive index layers (HI) and low refractive index layers (LI).
It is also known to interleave a sub-layer between the substrate and the first anti-reflection layer in order to improve abrasion and/or scratch resistance of said coating and its adhesion to the substrate.
Generally, classical anti-reflection (AR) coatings have a satisfactory heat resistance up to about 70° C. Above this temperature, cracks may appear in the AR stack, in particular at the surface of the substrate of the optical article, which damages the AR coating. In the present patent application, the temperature from which cracks are beginning to be observed in an article or coating is called the critical temperature (TC).
In the case of organic glass substrates (synthetic resin), deposition of the anti-reflection coating (optionally comprising a sub-layer) has to be performed through moderate temperature processes so as to avoid deterioration of the substrate. Taking such precaution is useless in the case of mineral glass substrates.
The consequence of a lower temperature treatment is, generally, in the case of organic glass substrates, a lower durability of the AR coating.
Moreover, organic glass substrates (either coated or uncoated) have a higher thermal expansion coefficient than inorganic materials constituting layers or sub-layers of the anti-reflection coating. The consequence is that they lead to articles which may develop high stress. Such stress may generate naked eye visible cracks or exfoliation in the AR coating upon increasing temperature.
This phenomenon is particularly noticeable when the organic substrate is based on diethylene glycol bis(allyl carbonate)monomers, episulfide monomers (materials having a refractive index n≧1.70), or polythiourethane (materials having a refractive index n equal to or higher than 1.60).
Different ways to improve the critical temperature of an optical article can be found in the literature.
US patent application 2005/0219724 describes an optical article coated with a multi-layer dielectric film, such as an anti-reflection coating, comprising alternate layers of high (TiO2) and low (SiO2 doped with a small amount of Al2O3, n=1.47) refractive indexes. According to this document, using SiO2/Al2O3 mixtures instead of SiO2 allows to decrease the stress in LI layers, and consequently the cracks appearance probability at the substrate surface.
Japanese patent H05-011101 (Hoya Corporation) describes the preparation of optical articles having initially a good thermal resistance, and which resistance to heating is maintained at a high level after several months. Both characteristics are obtained by the use of a SiO2/Al2O3 sub-layer having a refractive index of 1.48-1.52, interleaved between the substrate and a multi-layer AR coating comprising HI and LI layers. Some LI layers are composed of a mixture of Ta2O5+Y2O3+SiO2 and optionally Al2O3, leading to refractive indexes of 1.61-1.62, which is relatively high for a LI layer. The particular sub-layer improves the critical temperature of cracks appearance up to 100-105° C. at the initial stage.
Japanese patent H05-034502 is a variant of the latter Japanese patent in which the SiO2/Al2O3 sub-layer was replaced with a 3-layer sub-layer SiO2/Ta2O5/SiO2/Al2O3 mixture. The critical temperature of the optical article is raised to 95-120° C. at the initial stage with a diethyleneglycol bis(allyl carbonate) substrate.
Japanese patent H14-122820 (Seiko Epson Corporation) describes a hard-coated substrate coated with a SiO2 sub-layer having a physical thickness of 89-178 nm (optical thickness: 0.25-0.5λ at 520 nm) and a 4-layer anti-reflection coating ZrO2/SiO2/ZrO2/SiO2. According to this document, high critical temperatures can be reached by being able to balance coating thickness and stress between the layers of the various materials. However, the only parameter which was studied was the thickness of the sub-layer. Its thickness should be such that the ratio (sum of the physical thicknesses of the SiO2 layers, including the sub-layer)/(sum of the physical thicknesses of the ZrO2 layers) ranges from 2 to 3. Higher ratios are said to be undesirable because the durability of the AR coating is decreased. In fact, if the sub-layers having a physical thickness higher than or equal to 100 nm are not taken into account in the calculation, the LI/HI ratio is lower than or equal to 2 in the examples.
European patent application EP 1184685 (Hoya Corporation) describes an optical element having a plastic substrate and a λ/4-λ/2-λ/4 or λ/4-λ/4-λ/2-λ/4 AR film having a good heat resistance. The article is provided with a Nb (niobium metal) or SiO2 sub-layer in order to promote adhesiveness between the plastic substrate and the AR film. There are two conditions to achieve good heat resistance: i) the use of a specific layer of λ/2, which must be an equivalent film containing at least three layers and having a refractive index of from 1.80 to 2.40; ii) the even-numbered layer of the equivalent film must be a SiO2 layer.
European patent application EP 1184686 (Hoya Corporation) describes an optical element comprising a plastic substrate and, provided thereon in this order, a sub-layer comprising niobium metal (Nb) and an anti-reflection film. Said sub-layer is responsible for high adhesiveness between the plastic substrate and the anti-reflection coating, as well as excellent heat resistance and impact resistance. A SiO2 sub-layer is taught to decrease thermal resistance of the optical element.
A commercially available anti-reflection stack which is temperature resistant is also known. Neomultidiafal nMD, supplied by Essilor, is a 4-layer coating ZrO2/SiO2/ZrO2/SiO2 with respective thicknesses 12, 54, 28 and 102 nm. It is deposited in that order onto an ORMA® substrate (polycarbonate substrate from Essilor based on CR-39® monomer) coated with an anti-abrasion-coating. The resulting optical article has a critical temperature of 110° C. However, an optical article coated on both sides with this commercial anti-reflection stack has a mean luminous reflection factor Rv in the visible range (380-780 nm) as high as 2.3% (1.15% per face).